This invention relates to a thermal fixing developer material for electrophotography, and more specifically, to a thermal fixing developer material for electrophotography which is adapted for use as a main component of an electrostatic toner. The toner produced by using the developer material of the present invention exhibits excellent release properties upon thermal fixing, and therefore, is free from offset and contamination of carrier, photosensitive material, heat roller, and the like. The toner produced by using the developer material of the present invention also exhibits high blocking resistance.
In electrophotography, an electrostatic toner, which is a developer material for electrophotography, is used for developing a visible image from a latent image which has been created by application of a charge pattern on the surface of a photoreceptor. An electrostatic toner is a finely divided chargeable powder which may comprise a resin having dispersed therein a coloring agent such as carbon black and a pigment or the like. Electrostatic toners may be categorized into dry two-component toners which are used with a carrier such as iron powder or glass particles; wet toners which comprise a dispersion system in an organic solvent such as isoparaffin; and dry one-component toners having a magnetic finely divided powder dispersed therein.
The image which has been developed on the photoreceptor with the electrostatic toner is transferred onto a paper from the photo receptor, and on the paper, the image is fixed by means of heat or a vaporized solvent. In the case of an image which has been directly developed on a paper having disposed thereon a sensitizing layer, the image is fixed by means of heat or a vaporized solvent with no further transfer. Among various methods of fixing, fixing of an image using a heating roller is an advantageous method since it is a contact fusing at a high thermal efficiency which may enable a reliable fixing of the image even when a heat source at a relatively low temperature is used, and since such method is adequate for a high-speed reprography.
However, when a conventional electrostatic toner is used and the image is fixed onto a paper by bringing a heated body such as a heating roller in contact with the image, a phenomenon called offset is likely to be induced. In particular, in a high-speed reprography wherein the temperature of the heated body must be elevated to increase fixing efficiency and fixing rate, such offset phenomenon is more likely to be induced. In view of such situation, in the case of fixing an image created by one-component electrostatic toner using a heating roller, the roller surface is often impregnated with or fed with a silicone oil to thereby avoid such an offset phenomenon from being induced. Such countermeasure, however, may induce another problem that the roller may become susceptible for contamination.
In order to avoid the offset phenomenon, WO 93/16416 discloses an electrostatic toner comprising a wax wherein fixing of the image formed by the toner to the paper, namely, the strength of the fixed image is improved by using high molecular weight components, while suppressing the excessive increase in the softening point and the resulting poor low-temperature fixing induced by such use of the high molecular weight components. More illustratively, the increase in the softening point is suppressed by using a wax having a relatively high XR of from 1.80 to 2.50, namely, a relatively high content of comonomer-rich, low-melting components. The increase in the softening point is thus suppressed to thereby provide a wax of a high molecular weight having a relatively low softening point. In spite of the relatively good image fixing, temperature of low-temperature offset disappearance is rather high in such a toner due to the relatively high melting point and softening point. In addition, the high XR of from 1.80 to 2.50, namely, the relatively high content of .alpha.-olefin (comonomer) components in the low molecular weight range results in the inferior blocking resistance of the toner since the comonomer components act as sticky components.
For a binder which is one of the main component in the electrostatic toner, various thermoplastic resins have been employed. Among such thermoplastic resins which have been employed for the binder, low-molecular weight styrene-(meth)acrylate copolymers have the merits that they are highly chargeable; that they exhibit an excellent fixing owing to their adequate softening point(approx. 100.degree. C.); that cleaning of the photoreceptor is easy and the photoreceptor is less susceptible to contamination; that they are less hygroscopic; that they may be sufficiently mixed with carbon black which is the colorant; and that they may be readily pulverized. However, such conventional electrostatic toners prepared by using a low-molecular weight styrene-(meth)acrylate copolymer may still suffer from the offset phenomenon when they are used in the high-speed reprography.
In order to obviate the above-described problems, inclusion in the electrostatic toner of a polyolefin wax as a releasing agent has been proposed (JP-B-52-3304; JP-B-52-3305; JP-B-57-52574; JP-B-58-58664; JP-A-58-59455).
When the techniques described in the above-mentioned JP-B-52-3304 and the like are used, various problems other than the offset phenomenon are simultaneously induced. The toner having the polyolefin wax added thereto suffer from low blocking resistance, and such toner undergoes blocking in the toner cartridge, and in such a case, no toner would be fed to the photoreceptor. A filming phenomenon, that is, deposition of low crystalline substances of the wax on the carrier, photoreceptor, heating roller and the like, is also induced to adversely affect the formation of the electrostatic latent image or the electric charge of the toner to result in the disturbed image.